I'm a planner by nature, so I found it easier to make an itinerary - both to ensure we had reservations where/when needed, and to make sure we fit everything in that we wanted to do. I also would recommend an itinerary so you can make sure the things you are doing are in the same vicinity and you aren't criss-crossing the island. You also have to plan around traffic.
Diamond Head is best recommended early in the morning to avoid the heat and crowds. If it were me, I'd do it on a weekend. You'd hit traffic driving there in the morning on a weekday, and its best to spend the weekend days away from Aulani doing excursions because Aulani tends to be busier on the weekends with locals. Also keep in mind you'll likely be sweaty and in workout clothes (we were!) afterwards, so we didn't want to do much until we showered. I'd say you could check out Waikiki perhaps, but I wouldn't want to do anything that required nicer clothes or photo ops, for example.
Kualoa Ranch has tour schedules, and some are only offered once a day at certain times. And some tours are limited capacity. It can be a good 60-90 minute drive over there, depending on traffic, so this is one I would pre-book and have a day you want to dedicate to the windward side of the island. Kailua and Lanikai Beach aren't too far from here. We spent the morning at Kualoa Ranch and the afternoon at Kailua Beach.
Waimea Falls is in the North Shore, which would be a good day to check out Haleiwa and the Dole Plantation, if that's of interest.
As for dining at Aulani, it is recommended you have a reservation for Makahiki and Ama Ama. The character meals at Makahiki are popular, and we weren't able to move our dinner reservation because of that. We did breakfast and dinner at Makahiki and breakfast at Ama Ama and enjoyed them all. I don't think any of these meals are akin to CRT at WDW in terms of how difficult they are to book, but they can fill up especially depending on the time of year you'll be there.
Only other thing I would pre-book is a spa appointment, if that's of interest.